1. Field of the Invention
This invention concerns a method and apparatus for measuring temperature and, more specifically, it relates to the measuring of temperature at a higher accuracy in a high temperature and high pressure atmosphere such as in a hot isostatic pressing.
2. Description of the Prior Art
In various industrial processes, temperature measurement is one of the important procedures and it is often required to measure temperature information that occurs in the process and transmit this information to a distant place for the control and monitor the temperature condition of the relevant process. Particularly, in the hot isostatic pressing treatment, which is utilized such as for compression molding of ceramic powder or compacting the texture of the hard alloy, it is highly desired to measure the temperature and the distribution thereof in the high temperature and high pressure furnace at a desired accuracy and maintain the atmosphere in the furnace at an optimal condition based on the measured temperature information.
In order to meet such requirements, there have been proposed and developed various temperature measuring means including, for instance, thermocouples, as well as various types of thermometers such as gas thermometer, noise thermometer, fluid thermometer and radiation thermometer. However, these conventional temperature measuring means have not yet been satisfactory.
For instance, there is a difficulty in externally taking out the temperature information from the inside of the high temperature furnace. According to the experiment reported by Battle Memorial Institute, in which heat radiation from the inside of the furnace is taken out through an optical window made of sapphire attached to the structural component of a pressure vessel for the furnace (D. C. Carmiohael, P. D. Ownby, E. S. Hodge, "Hot Isotatic of Graphite" BML-1746 (1965)), the optical window was damaged due to the high temperature and high pressure to bring about an extremely dangerous state. The optical window was excessively heated by the radiation heat permeating therethrough which reduces the strength of the sapphire window and tended to cause destruction under the effect of the high pressure.
In addition, mounting of an optical window to the pressure vessel requires an forming opening in to the pressure vessel, which naturally reduces the pressure-resistance of the vessel. In the case of a hot isostatic pressing apparatus in which high pressure gas is sealed to the inside of the pressure vessel, destruction in the optical window or the pressure vessel may lead to fatal accidents or causeing other serious safety problems.
There is also another problem upon transmitting the temperature picked-up from the furnace. The temperature information after taken out by the foregoing thermocouples or radiation thermometer to the outside of the furnace is transmitted as electrical signals on a cable or like other line, which is extended to a distant place for the control and the monitor of the furnace, wherein noises due to electromagnetic induction or the likes will intrude into the transmission line.
In view of the above, the use of optical fibers for picking-up the radiation energy in the furnace as radiation light and transmitting the same through the optical path has recently been proposed and developed. For instance, Japanese Patent Laid-Open No. 129827/1981 proposes a thermometer using an optical fiber, in which radiation heat from a heat radiating target disposed at the incident end face of an optical fiber and the radiation light that is picked-up is transmitted through the optical fiber and then converted into electrical signals by means of an appropriate transducer for the measurement of the temperature. The problem in the structural damage at the temperature take-out portion of the furnace or the furnace body itself can be overcome and introduction of noises is also eliminated since the information is transmitted through as optical system.
However, if the heat radiating target is at an extremely high temperature, the optical fiber will be melted or devitrified thereby introducing inaccuracies. Additionally, it may become impossible to measure the temperature. Further, if the optical fiber is disposed in the high temperature and high pressure that exists in the inside of the furnace, the optical fiber would be excessively heated and dissolved by being exposed to the conduction and convection of the pressure medium at the high temperatures that exist in the furnace, which also causes a change in the optical deflection.
Further, the optical fiber disposed in the furnace may undesirably picks-up the peripheral radiation light in addition to the radiation light from the target irradiated to the incident end face of the optical fiber. Such peripheral light would cause, depending on the case, an error in the temperature measurement.
Furthermore, as a problem inherent to the use of the optical fiber, there exists the possibility of dust contamination of the incident end face of the optical fiber opposing to the target. Such a contamination will result in the decay in the amount of light incident to the optical fiber and cause noise and inaccuracy in the processed information. For instance, in the measurement for the radiation temperature using the optical fiber, if the incident end face of the fiber is contaminated to decrease the optical amount, the measured temperature would be lower than the actual temperature level.
In order to overcome the foregoing drawbacks in the prior art, the present invention use a continuous study for developing a method of measuring the temperature in the furnace of a hot isostatic pressing process using optical fiber(s) as a temperature pick-up means that can withstand the high temperature in the furnace and provide an accurate temperature measurement and which is free from the drawbacks due to the peripheral stray light or the comtamination of the incident end face of the fiber.